1. Field of the Invention
The present invention relates to a system and method for determining exhaust gas recirculation (EGR) flow for a multi-cylinder internal combustion engine.
2. Background Art
To improve performance, many internal combustion engines, particularly diesel engines, include a turbocharger to increase the oxygen density of the cylinder charge. Turbochargers use the engine exhaust gases to operate a turbine which in turn powers a compressor to compress intake air. Variable geometry turbochargers (VGT), which include variable nozzle turbochargers (VNT), are used to modify turbocharger characteristics over a broader engine operating range than possible with conventional turbochargers. Moveable intake or exhaust vanes (VNT) or a moveable turbine sidewall may be used to provide an appropriate amount of turbo boost pressure for current operating conditions and driver demanded engine torque.
Exhaust gas recirculation (EGR) has known advantages with respect to reducing emissions of nitrogen oxides (NOx) by reducing peak combustion temperatures within the engine cylinders during transient and steady-state operating conditions. A typical EGR system may include an EGR valve which diverts engine exhaust gases from the engine exhaust manifold to the engine intake manifold. The EGR valve may be an on/off type or modulating (proportional) type to regulate EGR flow and may be mounted at various places in the EGR circuit. In addition, an EGR cooler is often provided to cool the diverted exhaust gas with a rate measuring device to determine the amount or flow rate of diverted exhaust gas with appropriate tubing or piping connecting the exhaust side of the engine (exhaust manifold or turbocharger turbine) to the intake side of the engine (intake manifold or intake piping). The management of EGR flow is performed by an electronic engine control unit (ECU) in communication with the EGR valve and flow rate measurement device with the ECU providing closed loop control of EGR flow when operating conditions permit. The ECU will regulate the VGT and/or EGR valve based on input from the rate measurement device. Other derivatives of this arrangement may include a mixing device at the point of EGR gas entry to the inlet manifold and/or a venturi device to encourage a negative pressure differential across the engine as required to drive EGR flow from the exhaust side to the intake side of the engine. The ECU requires an accurate flow rate measurement and responsive control devices (EGR valve and/or VGT) to provide appropriate transient and steady state control to deliver expected performance and reduced emissions.
Optimization of engine emissions, performance, and fuel economy depends on the accuracy, response, and durability of the EGR system components, particularly the EGR measuring device. One method of EGR measurement provides a known restriction in the EGR circuit and measures the temperature and pressure drop across the restriction to determine the EGR rate. As the restriction increases, the accuracy of the rate calculation increases. However, the larger restriction inhibits the ability to flow EGR, is less accurate at low flow rates, and may be more susceptible to fouling. The restriction also requires the VGT to be controlled to provide more back pressure to drive the necessary amount of EGR which may impact the durability and reliability of the turbo machinery. In addition, the response is directly related to the temperature sensor response time which may be on the order of ten to fifteen seconds and makes transient control more difficult and complex. While software strategies can be applied to improve the response time by anticipating the rise in temperature, the harsh EGR environment may result in soot buildup over time on the sensing element which degrades the accuracy and performance.
A thermal anemometer may be used to measure EGR flow. Again, optimization of performance, fuel economy, and emissions is a function of the accuracy, response, and durability of the EGR rate measuring device. Anemometer accuracy is dependent on the specific heat of the gas being measured. For an EGR circuit, the specific heat of the exhaust gas can vary up to 10% over the operating range of a diesel engine the response of an anemometer device will be dependent on the temperature controller and the heater element design. Durability depends on the susceptibility to fouling and plugging.
The present invention provides a system and method for calculating EGR flow rate that includes a more accurate, responsive, durable, and/or reliable system and method for measuring or calculating EGR flow.
In one embodiment of the present invention, a method for determining EGR flow in a multi-cylinder internal combustion engine having a sensor for providing a signal indicative of flow of exhaust gas through an EGR circuit includes determining specific heat of the exhaust gas based on current engine operating conditions and determining EGR flow rate based on the determined specific heat and the signal provided by the sensor. A species tracking model is used to determine the amount of various species within the exhaust gas which may include oxygen, nitrogen, carbon dioxide, and water, for example, to determine the current specific heat. The current specific heat is compared to the specific heat for which the sensor has been calibrated with the difference used to adjust the sensor value and determine the EGR mass flow rate. A combustion model may be used in conjunction with the species tracking model to determine or estimate the amount of each species in the exhaust gas based on various engine operating parameters, such as fuel, air/fuel ratio, and turbo boost, for example.
The present invention provides a number of advantages relative to the prior art. For example, the present invention provides a more accurate, reliable, and durable EGR flow measuring device to optimize engine performance, fuel economy and emissions. Use of an anemometer-type flow measurement sensor eliminates any restriction in the EGR circuit otherwise required by a delta pressure sensor which may improve durability of turbo machinery by reducing required back pressure. The present invention provides a more accurate determination of EGR flow using an on-line determination of the current specific heat of exhaust gas passing through the EGR circuit using a species tracking model based on current engine operating parameters and conditions.
The above advantages, and other advantages, objects, and features of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.